Decking and Cladding

Deck flooring, facade cladding and interior panelling · Spacing, waste and fixings

Decking boards, cladding and panelling are often sold by the linear metre (lm) and used to cover surfaces. Calculating the correct quantity requires knowing the area dimensions, board width and either gap spacing or effective cover width — and adding a waste allowance for cutting and offcuts. This guide covers deck flooring, facade cladding and interior panelling. The three share the same quantity maths but have very different construction requirements — decking is a walked-on, free-draining horizontal surface; facade cladding needs a ventilated, drained cavity and weather detailing; interior panelling is protected. Use the figures here for estimating, but follow the correct build-up and detailing for each application (see the dedicated deck and facade guides).

The calculation principle

Required linear metres for boards with open gaps are calculated as follows:

lm = (Length × Width) ÷ (module width) [in m] × (1 + Waste%)

The module width is board width + gap for gapped or decking boards, or the effective cover width for overlapping/profiled cladding. The cover width already allows for the overlap, so for lap cladding divide by the cover width alone — do not also add the full board width.

Decking example: 6 m × 4 m deck, board width 120 mm, gap 6 mm, waste 10%:

lm = (6 × 4) ÷ (0.120 + 0.006) × 1.10 = 24 ÷ 0.126 × 1.10 ≈ 210 lm

Deck flooring

Choice of timber

• Pressure-treated pine: economical option for exterior use. Select the correct treatment/use class for the exposure conditions and follow the manufacturer's guidance for maintenance and finishing
• European larch: mid-range price with natural durability. It may be left untreated in some applications, but weathering, checking and maintenance needs vary with exposure and product guidance
• Thermally modified timber (thermo-wood): heat-treated pine or spruce — generally more dimensionally stable and more decay-resistant than untreated softwood. Follow the manufacturer's guidance for installation, fixing and maintenance
• Bangkirai / tropical hardwood: durability varies by species, grade and documentation. Use only clearly specified and responsibly sourced products, and follow the manufacturer's guidance for fixing and maintenance
• Composite decking (WPC): wood-plastic composite — generally low maintenance, but more expensive and often hotter in direct sun. Always follow the manufacturer's requirements for support, fixing and spacing

Board spacing (gaps)

• 5–8 mm: standard for decking — ensures drainage and ventilation beneath the boards
• Spacing as specified by the manufacturer: some composite and thermally modified timber systems use clips or proprietary fixing systems with defined spacing. Do not install boards tight unless this is explicitly supported by the product system
• New pressure-treated timber swells with moisture. In practice, around 5–6 mm is often used as a starting point, but the correct gap depends on moisture content, board profile and the manufacturer's guidance

Waste allowance

• 5%: rectangular area, boards running in one direction, minimal cutting
• 10%: standard recommendation for most decks
• 15–20%: diagonal laying pattern, many corners, curved edges

Facade cladding

Types of facade cladding

• Vertical board-on-board (feather edge or rebated): 22×98 / 22×148 mm — fixed vertically, joints covered with a cover strip. Traditional appearance
• Horizontal overlap cladding (shiplap / weatherboard): 19×120 / 19×148 mm — overlapping courses, fixed from the bottom upwards
• Horizontal rebated cladding: fixed horizontally with a ventilated cavity behind — good drainage performance

Lap and ventilation

• Horizontal lap cladding: overlap and exposed face width should follow the manufacturer's profile and installation guidance. Typical overlaps are often around 20–25 mm, but vary by profile and exposure
• The cavity behind cladding must be sufficient for drainage and drying. Use a documented ventilated solution and follow the manufacturer's installation guidance; in practice this is often around 23–25 mm
• The breather membrane and battening should form part of a documented drained and ventilated cladding build-up that ensures air circulation behind the cladding
• Preservative treatment/use class must match the exposure conditions. Above-ground exterior cladding is typically Use Class 3; Use Class 4 is generally reserved for ground contact or severe wet exposure. Follow local standards and manufacturer guidance

Waste allowance for cladding

• Horizontal: 8–12% (cuts around windows and corners)
• Vertical board: 10–15% (cuts at top/bottom and around openings)

Fixings

Decking screws

• Face screwing: 5.0×60 mm stainless steel or hot-dip galvanised (HDG) screws — often 2 screws per board per joist unless the manufacturer specifies otherwise. Fixing type, length, spacing and whether to face-fix or use hidden clips are product-specific (composite, thermally-modified and hardwood boards often need their own system and stainless fixings), so follow the board manufacturer's fixing schedule
• Typical consumption: 2 per board/joist crossing × number of joists under the boards. Example: 6 m board spanning 5 joists = 10 screws per board
• Pre-drill: always pre-drill hardwoods and thermally modified timber to prevent splitting

Hidden clips

• Used with grooved or profiled boards — the clip sits in the groove and is screwed to the joist
• Provides a cleaner, screw-free surface and defined spacing where the product system is designed for hidden fixing. Follow the manufacturer's guidance for clips, joist spacing and board profile
• Typically 1–2 clips per board per joist — check the manufacturer's recommendation

Reference table

Common mistakes

• ✗Forgetting the gap in the calculation — you order too few boards and run short at the end
• ✗Too little waste allowance — diagonal laying and many corners produce 15–20% offcuts, not 5%
• ✗Decking installed tight against the house wall — cladding and panels need a ventilated cavity behind
• ✗Wrong screws (black steel) on decking — corrodes quickly and leaves ugly stains in the timber
• ✗Not pre-drilling hardwood and thermo-wood — boards split from the end
• ✗Bottom edge of cladding too close to the ground — splashback and moisture wick up and rot the boards. A clearance such as ~200–300 mm above finished ground is a common guideline (often quoted as ~250 mm), but the exact figure depends on the detail, exposure and local guidance — keep the bottom edge well clear of ground, paving and the snow line
• ✗Installing cladding without painting end grain and back faces first — end grain absorbs moisture fastest and is where rot starts

Calculate your materials

Use the calculator to find the exact number of linear metres and cost: